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Indicating Character Encoding and Language for HTTP Header Field Parametersgreenbytes GmbHHafenweg 16MünsterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/Applications and Real-Time
HTTPHTTPheader field parameterinternationalization
By default, header field values in Hypertext Transfer Protocol (HTTP)
messages cannot easily carry characters outside the US-ASCII coded
character set. RFC 2231 defines an encoding mechanism for use in
parameters inside Multipurpose Internet Mail Extensions (MIME) header
field values. This document specifies an encoding suitable for use in
HTTP header fields that is compatible with a simplified profile of the
encoding defined in RFC 2231.
This document obsoletes RFC 5987.
Discussion of this draft takes place on the HTTPBIS working group mailing list
(ietf-http-wg@w3.org), which is archived at .
Working Group information can be found at ; source code and issues
list for this draft can be found at
.
The changes in this draft are summarized in .
Use of characters outside the US-ASCII coded character set ()
in HTTP header fields () is non-trivial:

The HTTP specification discourages use of non-US-ASCII characters in field
values, placing them into the "obs-text" ABNF production ().

Furthermore, it stays silent about default character encoding schemes for
field values, so any use of non-US-ASCII characters would need to be specific
to the field definition, or would require some other kind of out-of-band information.

Finally, some APIs assume a default character encoding scheme in order to map from the octet sequences (obtained from the HTTP message) to character sequences:
for instance, the XMLHttpRequest API () uses the Interface Definition Language type "ByteString",
effectively resulting in the ISO-8859-1 character encoding scheme
being used.

On the other hand, RFC 2231 defines an encoding mechanism for parameters
inside MIME header fields (), which, as opposed to
HTTP messages, do need to be sent over non-binary transports.
This document specifies an encoding suitable for use in HTTP header fields
that is compatible with a simplified profile of the encoding defined in RFC 2231.
It can be applied to any HTTP header field that uses the common "parameter"
("name=value") syntax.
This document obsoletes and
moves it to "historic" status; the changes are summarized
in .
Note: in the remainder of this document, RFC 2231 is only referenced
for the purpose of explaining the choice of features that were adopted; they
are therefore purely informative.
Note: this encoding does not apply to message payloads
transmitted over HTTP, such as when using the media type "multipart/form-data"
().
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in .
This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in
. The following core rules are included by
reference, as defined in :
ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and
LWSP (linear whitespace).
This specification uses terminology defined in ,
namely: "character encoding scheme" (below abbreviated to
"character encoding"), "charset"
and "coded character set".
Note that this differs from RFC 2231, which uses the term "character set"
for "character encoding scheme".
RFC 2231 defines several extensions to MIME. The sections below discuss
if and how they apply to HTTP header fields.
In short:
Parameter Continuations aren't needed (),Character Encoding and Language Information are useful, therefore a simple subset
is specified (), andLanguage Specifications in Encoded Words aren't needed (). defines a mechanism that
deals with the length limitations that apply to MIME headers. These
limitations do not apply to HTTP ().
Thus, parameter continuations are not part of the encoding defined by this
specification.
specifies how to embed
language information into parameter values, and also how to encode
non-ASCII characters, dealing with restrictions both in MIME and HTTP
header field parameters.
However, RFC 2231 does not specify a mandatory-to-implement character encoding,
making it hard for senders to decide which encoding to use.
Thus, recipients implementing this specification &MUST; support the
"UTF-8" character encoding .
Furthermore, RFC 2231 allows the character encoding information to be left out.
The encoding defined by this specification does not allow that.
The presence of extended parameter values usually is indicated by a parameter name
ending in an asterisk character. Note however that this is just a convention,
and that it needs to be explicitly specified in the definition of the header
field using this extension (see ).
The ABNF for extended parameter values is specified below:
ext-value = charset "'" [ language ] "'" value-chars
; like RFC 2231's <extended-initial-value>
; (see )
charset = "UTF-8" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in
; except that the single quote is not included
; &SHOULD; be registered in the IANA charset registry
language = <Language-Tag, see >
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
The value part of an extended parameter (ext-value) is a token that consists
of three parts:

the &REQUIRED; character encoding name (charset),

the &OPTIONAL; language information (language), and

a character sequence representing the
actual value (value-chars), separated by single quote
characters.

Note that both character encoding names and
language tags are restricted to the US-ASCII coded character set, and are matched
case-insensitively (see and
).
Inside the value part, characters not contained in attr-char are
encoded into an octet sequence using the specified character encoding. That octet
sequence is then percent-encoded as specified in .
Producers &MUST; use the "UTF-8" () character encoding.
Extension character encodings (mime-charset) are reserved for future use.
Note: recipients should be prepared to handle encoding
errors, such as malformed or incomplete percent escape sequences, or
non-decodable octet sequences, in a robust manner. This specification
does not mandate any specific behavior, for instance, the following
strategies are all acceptable:
ignoring the parameter,stripping a non-decodable octet sequence,substituting a non-decodable octet sequence by a replacement
character, such as the Unicode character U+FFFD (Replacement Character).
The
RFC 7230 token production ()
differs from the production used in RFC 2231 (imported from )
in that curly braces ("{" and "}") are excluded. Thus, these two
characters are excluded from the attr-char production as well.
The <mime-charset> ABNF defined here differs from
the one in in that it does
not allow the single quote character (see also RFC Errata ID 1912 ). In practice, no character encoding names
using that character have been registered at the time of this writing.
For backwards compatibility with RFC 2231, the encoding defined by this
specification deviates from common parameter syntax in that the
quoted-string notation is not allowed. Implementations using generic parser
components might not be able to detect the use of quoted-string notation and
thus might accept that format, although invalid, as well.
did require support for ISO-8859-1 (),
too; for compatibility with legacy code, recipients are encouraged to
support this encoding as well.
Non-extended notation, using "token":
foo: bar; title=Economy
Non-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"
Extended notation, using the Unicode character U+00A3 ("£", POUND SIGN):
foo: bar; title*=utf-8'en'%C2%A3%20rates
Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Also, note
that the space character was encoded as %20, as it is not contained in
attr-char.
Extended notation, using the Unicode characters U+00A3 ("£", POUND SIGN)
and U+20AC ("€", EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Likewise,
the Unicode euro sign character U+20AC was encoded into the octet
sequence E2 82 AC, then percent-encoded. Also note that HEXDIG allows
both lowercase and uppercase characters, so recipients must understand
both, and that the language information is optional, while the character encoding is not.
extends the encoding
defined in to also support language specification
in encoded words.
RFC 2616, the now-obsolete HTTP/1.1 specification, did refer to RFC 2047
(). However, it wasn't clear
to which header field it applied. Consequently, the current revision of
the HTTP/1.1 specification has deprecated use of the encoding forms defined in
RFC 2047 (see ).
Thus, this specification does not include this feature.
Specifications of HTTP header fields that use the extensions defined
in ought to clearly
state that. A simple way to achieve this is to normatively reference
this specification, and to include the ext-value
production into the ABNF for specific header field parameters.
For instance:
foo = token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see draft-ietf-httpbis-rfc5987bis, >
Upon publication as RFC, the string "draft-ietf-httpbis-rfc5987bis" needs to be replaced with the RFC name, and this comment needs to be removed.Note: The Parameter Value Continuation feature defined in
makes it impossible to
have multiple instances of extended parameters with identical names,
as the processing of continuations would become ambiguous. Thus, specifications
using this extension are advised to disallow this case for compatibility
with RFC 2231.
Note: This specification does not automatically assign a new
interpretation to parameter names ending in an asterisk. As pointed out
above, it's up to the specification for the non-extended parameter to "opt
in" to the syntax defined here. That being said, some existing
implementations are known to automatically switch to the use of this
notation when a parameter name ends with an asterisk, thus using parameter
names ending in an asterisk for something else is likely to cause
interoperability problems.
requires that protocol
elements containing human-readable text are able to carry language information. Thus, the ext-value
production ought to be always used when the parameter value is of textual
nature and its language is known.
Furthermore, the extension ought to also be used whenever the parameter value
needs to carry characters not present in the US-ASCII ()
coded character set (note that it would be unacceptable to define a new parameter that
would be restricted to a subset of the Unicode character set).
Header field specifications need to define whether multiple
instances of parameters with identical names are allowed, and
how they should be processed. This specification suggests that a parameter using the
extended syntax takes precedence. This would allow producers to use both
formats without breaking recipients that do not understand the extended syntax
yet.
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20rates
In this case, the sender provides an ASCII version of the title
for legacy recipients, but also includes an internationalized version for
recipients understanding this specification -- the latter obviously
ought to prefer the new syntax over the old one.
The format described in this document makes it possible to transport
non-ASCII characters, and thus enables character "spoofing" scenarios,
in which a displayed value appears to be something other than it is.
Furthermore, there are known attack scenarios relating to decoding UTF-8.
See for more information on
both topics.
In addition, the extension specified in this document makes it possible
to transport multiple language variants for a single parameter, and such use might allow spoofing attacks, where
different language versions of the same parameter are not equivalent.
Whether this attack is useful as an attack depends on the parameter
specified.
There are no IANA Considerations related to this specification.
ASCII format for network interchangeKey words for use in RFCs to Indicate Requirement LevelsIANA Charset Registration ProceduresUTF-8, a transformation format of ISO 10646Uniform Resource Identifier (URI): Generic SyntaxTags for Identifying LanguagesAugmented BNF for Syntax Specifications: ABNF
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingHypertext Transfer Protocol (HTTP/1.1): Semantics and ContentErrata ID 1912, RFC 2978RFC ErrataInformation technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1International Organization for StandardizationMultipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextMIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and ContinuationsIETF Policy on Character Sets and LanguagesHypertext Transfer Protocol -- HTTP/1.1Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field ParametersWeb LinkingUse of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)Terminology Used in Internationalization in the IETFReturning Values from Forms: multipart/form-dataHTTP Digest Access AuthenticationHTTP Authentication Extensions for Interactive ClientsXMLHttpRequestWhatWG
This section summarizes the changes compared to :

The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters".

The introduction was rewritten to better explain the issues around non-ASCII characters in field values.

The requirement to support the "ISO-8859-1" encoding was removed.

The document does not attempt to re-define a generic "parameter" ABNF anymore (it turned out that
there really isn't a generic definition of parameters in HTTP; for instance,
there are subtle differences with respect to whitespace handling).

A note about defects in error handling in current implementations was removed, as it wasn't accurate anymore.

The encoding defined in this document currently is used in four different
HTTP header fields:

"Authentication-Control", defined in ,

"Authorization" (as used in HTTP Digest Authentication, defined in ),

"Content-Disposition", defined in , and

"Link", defined in .

As the encoding is a profile/clarification of the one defined in
in 1997, many user agents already supported it for
use in "Content-Disposition" when got published.
Since the publication of , three more popular desktop
user agents have added support for this encoding; see
for details. At this time, the current versions of all major desktop
user agents support it.
Note that the implementation in Internet Explorer 9 does not support the
ISO-8859-1 character encoding; this document revision acknowledges that UTF-8 is
sufficient for expressing all code points, and removes the requirement
to support ISO-8859-1.
The "Link" header field, on the other hand, was more recently specified
in . At the time of this writing, no User Agent
except Firefox supported the "title*" parameter (starting with release 15).
defines the "username*"
parameter for use in HTTP Digest Authentication. At the time of writing, no
User Agent implemented this extension.
Only editorial changes for the purpose of starting the revision process
(obs5987).
Resolved issues "iso-8859-1" and "title" (title simplified).
Added and resolved issue "historic5987".
Added issues "httpbis", "parmsyntax",
"terminology" and "valuesyntax".
Closed issue "impls".
Resolved issue "terminology".
In , pull historical notes into a separate subsection.
Resolved issues "valuesyntax" and "parmsyntax".
Update status of Firefox support in HTTP Link Header field.
Update status of Firefox support in HTTP Link Header field.
Update status with respect to Safari 6.
Started work on update with respect to RFC 723x.
Editorial changes; introducing non-ASCII characters into author's
address, acknowledgements, and examples.
Removed mention of RFC 2616 from Abstract and Introduction.
Reference RFC 20 for US-ASCII.
Do not attempt to define a generic parameter ABNF; just concentrate on the
parameter value syntax.
RFC 2388 -> RFC 7578.
Expand on the motivation (see ).
Mention RFC 7616 in implementation report.
Fixed one editorial issue. Updated XHR reference.
Fixed : use of now undefined term "parmname".
Include WG into Acknowledgements for this revision.
Mention RFC 5987 in the abstract ().
Mention RFC8053 in Implementation Report.
Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF details,
to Graham Klyne and Alexey Melnikov for general review, to
Chris Newman for pointing out an RFC 2231 incompatibility, and to
Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger for implementer's feedback.
Furthermore thanks to the members of the IETF HTTP Working Group for
the feedback specific to this update of RFC 5987.